The invention relates to a method for processing high contrast silver halide photographic materials.
The vast majority of commercially available developers for black and white films are based on hydroquinone. However, the use of hydroquinone is suspect from an ecological point of view. It can have allergic effects and some countries are proposing to list hydroquinone as a suspect carcinogen. Developing agents not showing these disadvantages are ascorbic acid and related compounds [see, for example, P. Meeuws et al., (Agfa Gevaert), Research Disclosure, March 1995, p. 185].
Ascorbic acid has been recognized as a developing agent for some time, and, although less active than hydroquinone or catechol, the photographic literature reports it will develop a conventional emulsion at high pH [see, for example, W. E. Lee and E. R. Brown in The Theory of the Photographic Process, 4th ed., ed. T. H. James, Macmillan, New York and London, 1977, chapter 11]. However, the number of instances of the commercialization or useful employment of an ascorbic acid-based developer are relatively few. An ascorbic acid-based developer suitable for hydrazine containing high contrast films has been described in U.S. Pat. No. 5,236,816. Also, U.S. No. 5,474,879 describes an ascorbic acid developer particularly suitable for radiographic films.
For many years the very high contrast photographic images needed in the graphic arts and printing industries were obtained by developing a xe2x80x9clithxe2x80x9d emulsion (usually high in silver chloride content) in a hydroquinone, low sulfite, xe2x80x9clithxe2x80x9d developer by a process known as infectious development. High contrasts were achieved. However, such low sulfite developers are inherently unstable and are particularly inappropriate for machine processing.
Machine processing of graphics materials was achieved by the use of so-called xe2x80x9crapid accessxe2x80x9d high contrast materials which have a lower scale (or toe) contrast below 3 and typically about 2, good process latitude and good process stability. Such materials are easy to use but this is at the expense of noticeably reduced dot quality (i.e. a so-called xe2x80x9csoftxe2x80x9d dot) and hence are not suitable for users requiring the highest of dot qualities. Rapid access film materials are, however, widely accepted and used and are in daily use alongside nucleated products described immediately below.
To achieve the high image quality obtainable from lith processing but with a stable process, emulsions containing nucleating agents, for example hydrazides, have been used and processed in a high pH (xcx9c11.5) developer with conventional amounts of sulfite, hydroquinone and co-developer. A further refinement in the area of high contrast materials was the introduction of a lower pH process (typically xcx9c10.4) using hydrazides active at this pH together with the use of an incorporated contrast booster compound, such as an amine. Both these processes provide half-tones with a so-called xe2x80x9chardxe2x80x9d dot.
However, such materials incorporating nucleators and booster compounds are not ideally desirable because the process sensitivity is still substantially worse than that obtained with the rapid access process.
The infectious process phenomenon of xe2x80x9cco-developmentxe2x80x9d [R. Beels and F. H. Claes, The Journal of Photographic Science, 1975, 22, 23] is defined as the tendency for unexposed silver halide grains with no latent image to develop if they are in the near vicinity of developing grains which are fogged. No spectral sensitization is described. The extent of the co-development reported has been insufficient to make this little more than an interesting observation.
EP-A-0 758 761 discloses that when an imagewise exposed silver halide layer having both spectrally sensitized and non-spectrally sensitized silver halide grains, a high silver:gel ratio, and containing an appropriate amine, its density can be enhanced by the co-development effect to give a substantial density gain enabling the production of a high contrast photographic material which does not contain a nucleating agent. Advantages of the enhanced co-development film were cited to be: the lack of a nucleating agent and the use of less silver, gelatin and sensitizing dye to obtain improved contrast/image quality, lower post-process dye stain through reduced dye laydown and reduced cost. In addition the absence of nucleating agents meant the film was free of so-called xe2x80x9cpepper fogxe2x80x9d.
Although efficacious, the use of enhanced co-development films has some limitations and drawbacks. In spite of the fact that such materials are relatively process insensitive in hydroquinone-based developers, there remains some process sensitivity which it would be advantageous to reduce or eliminate.
Additionally, in hydroquinone-based developers where, through poor process management, the pH has been allowed to fall below 10.0 and towards 9.5, the dot quality becomes less acceptable, with ragged high contrast edges which do not give good results on contacting to, for example, a contact film or printing plate.
Finally, at pH values of 10.0 or lower there appears in non-image areas a development of silver specks which are considered to be undesirable.
A method of forming a photographic image in an imagewise exposed high contrast photographic material free from nucleating agents comprising a support bearing a silver halide emulsion layer comprising silver halide grains wherein at least 10% of the silver halide grains are spectrally sensitized and a hydrophilic colloid having a silver:hydrophilic colloid ratio above 1,
the method comprising developing the photographic material in a developer comprising a primary ascorbic acid developing agent at a pH no greater than 10.5, the developer being free of hydroquinone.
The combination of the co-development photographic material and an ascorbic acid-based developer allows for all the previous advantages of the material to be retained, such as a reduction in the amount of sensitizing dye used compared to conventional graphics films thus providing low post-process dye stain and lower product cost, while extending to it the ability to obtain a high contrast result with an ecologically advantageous developer. Unexpectedly, the combination allows process insensitivity greater than that obtained with conventional hydroquinone-based developers. In addition the dot quality from the enhanced co-development photographic material in the ascorbic acid-based developer does not deteriorate as the pH falls. Finally, the unwanted development of silver specks in unexposed areas of the photographic material is significantly and surprisingly reduced by the method of the invention compared against the method in which the same material is developed in a hydroquinone-based developer.